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"How come on Star Trek everybody's private parts are the same? No alien lady ever told Captain Kirk, 'Hey, get your thing out of my nose!'"
—Rajesh Koothrappali, The Big Bang Theory
In works involving non-humans (whether it be anthropomorphic beings, mythological creatures, or aliens) having sex, there's a tendency for everything to still use a human style sexual system (most visible in the penises). Everything will have sex organs based around humans of a size that allows the owners to copulate with each other as the Plot demands. Often, this manifests itself as a differently colored human penis/vagina, but even when some design consideration is given towards making them look more like the species the creature is based on, they still have a tendency towards human proportions. And human stimulation—it's believed that primates and dolphins are the only creatures that copulate when not fertile.
In Real Life, the genitalia of even closely related species often have significant differences, making cross-species coupling impossible (in most cases, at least), even if they both follow the "Sword and Sheath" model.
- Rather frighteningly to anyone who accidentally stumbles upon the wrong web link, this is absolutely an area Rule 34 has not missed in Real Life... most commonly with a few specific classes of mammals.
- On the other hand, some do use physically accurate organs in bestiality or furry porn. Sometimes additionally drawing attention to differences.
Anime & Manga
- Bondage Fairies exemplifies this Trope like nothing else, where everything from small ladybugs to birds all appear exactly like the real animals... except for cut and paste penises that are all large enough (or, in some cases, small enough) to fit comfortably inside the vaginas (and other orifices) of the fairies.
- Averted in the porn Anime The Elven Bride, in which the titular heroine is at a markedly different state of sexual development than her husband even though they're the same age (her vagina is too small and underdeveloped for them to have sex normally). Cue the search for a legendary lubricant that should make it possible.
- Played completely straight in Macross (and by extension, the Macross segment of Robotech): Humans and Zentraedi can have children apparently without any sort of outside aid, as seen when Max and Milia have a daughter shortly after getting married. Justified in Macross Continuity by the fact that both races were created by the Protoculture.
Comics — Books
- XXXenophile switches between this and Exotic Equipment.
- Ironwood uses this Trope, although there it is mostly a case of the female genitalia of the fantasy races being compatible with human male equipment. The males occasionally have trouble finding the right spot but, once they do, there are no problems doing the deed.
- Tank Girl routinely gets it on with mutant/genetically engineered kangaroo Booga. His... equipment... is human.
Films — Live-Action
- District 9 mentioned, but didn't show, "interspecies prostitution". One wonders as to the mechanics, as the Prawn are naked for the whole movie and don't have visible external genitalia. They are shown to reproduce by laying eggs.
One assumes, given how grossed out everyone is by the idea, that it is just about as natural and comfortable as having sex with a chicken (which people actually have been known to do). In other words, it's taboo even amongst deviants but there's some people who are just too damned kinky and/or curious not to try.
- A deleted scene mentions that they're hermaphrodites.
- In Galaxy Quest The human-looking aliens are actually some sort of squid thing under their holograms. Chen and Laliari don't seem to care.
Guy: That ain't right!!!
- SF author John Varley absolutely loves this Trope:
- The Gaea Trilogy has the Titanides, an alien species who are sexually compatable with humans because they were deliberately engineered to be sexually compatible with humans.
- Steel Beach has the ULTRA tingle system, which gets around the whole genital compatibility issue entirely by using wireless modems for sex.
- The actor protagonist of The Golden Globe equipped himself with a penis that can be turned inside out for use as a vagina, primarily because it doubles his chances of getting roles.
- Finally, the novel The Opiuchi Hotline has literal male to female universal adaptors that allow characters who go in for radically modified genitals to "dally" with their more conservative counterparts.
- In The California Voodoo Game, two conspirators meet at a sleazy hotel called the Mate & Switch. Each room is equipped with sensory-simulation bodysuits that let the wearer experience the sensations of being the opposite gender, making each a literal Male-to-Female Universal Adaptor.
- Simply glossing over this (very Edwardian, of course) doesn't mean it's not there; John Carter of Mars and his red Martian princess Dejah Thoris. Would not her biological equipment for laying eggs cause at least a bit of a shock for our Earthly hero?
- An interesting version occurs in the Superhero novel Soon I Will Be Invincible. The heroine Damsel is the daughter of a human male and a Green-Skinned Space Babe, however, she was bred in a laboratory because her parents were not compatible with each other. Her mother wasn't even a mammal.
- Subverted in Stationery Voyagers, in which there is some discussion about which races of Stationeries are compatible with each other and which ones are not. (Pencils are only barely compatible with Gel Pens, and are otherwise not very compatible with anything. They even have difficulty being compatible with each other, so they're a slowly-dying-out race.) Eraser-Man is only compatible with Erasers, and they're functionally extinct. Whiteouts are only compatible with Whiteouts. Gel Pens, Pens, Markers, etc., can interbreed with apparently no problems. Since it's a divine curse that keeps them from all being human, it follows its own logic. Humans and Mosquatlons can interbreed, but it's not considered good for a Mosquatlon's mental health to outlive your first love by 500 years. Aviatets, however, are only compatible with Aviatets.
Worse, a Stationery penis is short and thin (and usually concealed behind a skin shield,) and has fewer nerve endings than in a human. Likewise, a Stationery vagina has very few nerve endings. They need drugs to get horny. Due to their design (namely, not having legs,) they are also very limited in the number of positions they can experiment with. Actually inspires a lot of their Sex Is Evil attitude; since outside of procreation, it really is a complete waste of time for them.
- Played with in James Patrick Kelly's Look into the Sun: The protagonist is sent on an interstellar trade mission that requires him to be transformed into an alien species that just happens to use sex as a bonding and bargaining tool. Fortunately, he's also been equipped with a neural AI that can translate all of the alien physical sensations into the human ones he's familiar with... until the AI gets pettish mid-screw and refuses to translate any further, leaving him to cope with his new Bizarre Alien Biology on his own, such as the sudden realization that his delicate, flower-like genitalia means the females have been taking the dominant role during copulation all along.
- In West of Eden, Yilanè have reptilian genitalia, including hemipenes on the males (who also function as Mister Seahorse). However, the female genitals are compatible enough with human equipment that Vaintè and Kerrick are capable of having plenty of sex while the latter is the former's personal slave. It should be noted, however, that Vaintè is insane both by human standards and those of her own race, and Kerrick was raised by Yilanè and has (at this point) an incomplete and screwed-up understanding of human sexuality. When Kerrick returns to humanity, he starts looking back on his time with Vaintè in disgust and horror.
- In the Babylon 5 Spin-Off Crusade, Captain Gideon accidentally acquires a data crystal containing Pak'ma'ra/human porn. The ship's doctor walks in and comments that this shouldn't work. (The Pak'ma'ra are humanoid, but closer to Starfish Aliens than Rubber Forehead Aliens with tentacles around the mouth and a hump that is the female of their species) Then they bend their necks ninety degrees with a look of horror on their faces. Gideon remarks that technology is a wonderful thing.
- Also encountered in the original Babylon 5 series, which unexpectedly describes Centauri genitalia quite explicitly and as being mostly (if not entirely, barring near-perverse levels of creativity) incompatible with comparable human anatomy. Although looking like they should be just simple Rubber Forehead Aliens (they look like sharp-toothed humans with strange hairdos), genetically they have nothing in common with us. To the point they have 6 tentacles on the male and 6 vaginas on the female's backs. Their equivalent of kissing is penetration with just 1. Oh and the tentacles are prehensile and Londo gets caught using one to cheat at poker...
- In point of fact, it isn't terribly difficult to imagine Centauri males having sex with humans (of either sex), although he'd need at least two or three human partners to get maximum stimulation. The details of the Centauri female reproductive anatomy being somewhat less clear, we can't really be sure whether human men would be able to do it with a Centauri woman... or if she would care to do it if they could. In either case...well... just read the next bit about G'Kar.
- The matter is clarified in the Centauri Republic supplement for the Babylon 5 RPG. Centauri women have six vagina-analogues on their backs, each with its own ovary. To get pregnant, all six part-eggs must be fertilized and combine, a process which make genetic defects extremely rare among Centauri. In many ways, they're the least human-like of the various races. At least the oxygen-breathers.
- G'Kar (before his spiritual transformation) is seen as being also a bit of pervert, being into Human women as well as at least one Centauri female. His species, although looking reptilian, are marsupials, including keeping their young in a pouch. As Na'Toth said in her first appearance, "Far be it from me to ask how anything gets in your bed..."
- A throwaway line about "what are these cramps I'm getting" when Delenn becomes a half-human hybrid implies that her species, at least, has a rather different reproductive system than humans. On the other hand, that could just mean that her species' females don't menstruate... which is true of most mammals here on Earth.
- Most alien species, at least those encountered by Starfleet, seem to be sexually compatible with humans in the Star Trek universe. Even those that shouldn't be, like the Cardassians (which, according to Canon, supposedly evolved from reptile-like ancestors, but are capable of mating and reproducing with mammalian humanoids). In an unexpected subversion of this, it is noted that Klingons, Vulcans and humans, while sexually compatible, have a difficult time producing hybrid offspring "without help". In the Next Generation episode "The Chase" it is revealed long before humans, Klingons, Vulcans, Cardassians, and other races came into being, there was a dying race who spread their genetic material to developing worlds in hopes of their legacy continuing. This is likely the reason why there is such genetic compatibility.
- Implied in the Star Trek: Voyager episode "The Disease", when Ensign Harry Kim has a passionate affair with beautiful humanoid alien Tal.
Kim: I've got to be honest, I wasn't expecting something so different.
- Subverted in the 1980s miniseries V, in which a reptilian humanoid is required to undergo surgical alteration (and make no mistake, even the simple act of implying as much was pretty racy for early 1980s primetime!) before mating with a human female. The novelization goes a little further than the TV show did, noting that the human subject of the experiment — although unaware of how alien her lover is — senses that something is wrong.
- In the recent re-imagining, it is perfectly possible for a Visitor to have sex with a human, as evidenced by two of the show's interspecial couples. One of them, in fact, results in a Half-Human Hybrid, something the aliens thought impossible given the two race's diverse origins.
- Subverted in Lexx with Stanley and Lyekka. Although they would have sex if given the chance, Lyekka is a highly evolved plant and has no genitalia. There's also the issue of Lyekka's entire raison d'etre being to seduce other species by appearing in a form desirable to them for the purpose of EATING THEM. Despite her affections for Stanley, she is still just a plant that might just eat him if they ever tried doing it.
- Hilariously Conversed about in The Big Bang Theory: After the gang speculate on how Amanda and Sarek concieved Spock, considering their difference in species, Raj muses on how in Star Trek everybody's private parts seem to be the same: "No alien lady ever told Captain Kirk 'Hey, get your thing out of my nose!'"
- Presumably the case in Farscape, at least most of the time, though you never actually see the genitals. Implied by Chiana and D'Argo, for instance, given that their sexual positions seem very human-esque and even though they are very different species explicitly stated as not genetically compatible, they still manage to have lots and lots of sex.
- D'Argo (a luxan) had a hybrid son (Jothee) with a sebacean (augmented human) wife. Jothee also lots of human-like sex with Chiana. Zhann (a blue humanoid plant) at one point states that Luxans 'make excellent lovers', implying the two can have sex, but not necessarily reproduce.
- Aeryn (a sebacean) has a child with Crighton (a human), even though she details many large differences in her reproductive biology.
- Exalted averted this trope with an April Fools sex supplement. One of the artifacts in the PDF is called the Many-Stalked Flower of Delights, which allows beings of incredibly different physiology to enjoy the pleasures of each other's company. The creator of the artifact was an Eclipse who apparently fell in love with a mountain. Not the god of the mountain, the mountain itself. The first use of the item caused a volcanic eruption. Of course, the Many-Stalked Flower of Delights is a Male-to-Female Universal Adaptor, but it's the opposite of this trope.
- Apparently in Mass Effect human and Quarian genitals are similar enough that sex can occur normally once you get around the problems inherent in cross-species fluid contact.
- Don't forget the other races; heck, Mordin has pamphlets on cross-species intercourse for whatever Romance Sidequest you pursue. Drell, turian, quarian, he's got you covered.
- It's not made clear if all the parts line up precisely. The pamphlets are necessary to achieve proper positioning and to hit the right erogenous zones. All we really know is that the respective parts are in the same general area on the humanoid species.
- It's also for health safety. Quarians with their atrophied immune system are only an extreme case. There are also all kinds of allergies to the body chemistries of other species.
- The Web Comic Dreamwalk Journal does this with human-insect/arachnid hybrids, although everything seems to be scaled up quite a bit, even for visiting humans.
- Justified in that, according to Word of God, the hybrids and their sexual customs were all created by human genetic engineers centuries or millennia in the past.
- Averted in Order of the Stick with lizardfolks, who have a genital sheath and hemipenes, like most reptiles. Though this doesn't seem to stop interspecies prostitution, at least with the females.
- Undead seem to be able to breed in both Grim Tales from Down Below and Dungeon & Denizens; detailed explanations on what male "biophiliacs" (including skeletons with no flesh) do for sex and procreation:  . So while it goes unmentionned, they MUST use some adaptator, right?
- Spoiler for Grim Tales from Down Below: Yes. It's called another dude, and an abortion clinic. Think about that for a minute — how else did you think a skeleton would have a dead baby?
- Jay Naylor varies use of this trope. Sometimes male organs are human-like, other times (mostly with equines and canines) they are anatomically close to the Real Life animals. Females, however, universally have human-like vaginas.
- The title trio of the American animated series Aqua Teen Hunger Force have all displayed sexual interest in and claim to have had sex with human females at one point or another, despite the apparent lack of genitalia (and the fact that all three are a giant, anthropomorphic food items — a box of fries, a milkshake and a meatball, respectively). And yet two have vaguely phallic appendages and the third can change his form a bit...
- However, it should be noted that Shake is a huge goddamn liar, and Frylock mentions at several points that Shake doesn't have a penis. When Shake claims to have had sex one time, Frylock's response is "When? How? With what?" It's repeatedly mentioned that Frylock is a virgin, but this seems to be attributed more to his nerdiness than any physiological shortcomings.
- In Ben 10 Alien Force, the main source of superpowers is revealed to be having alien ancestry, which includes, among other things, energy beings (although granted said energy being could assume human form) and an alien made out of living fire (which is a bit harder to explain). Plus there's the whole human marrying Mud Monster story from the previous series....